The invention disclosed and claimed herein generally pertains to an improved spiral scanning technique for magnetic resonance (MR) imaging. More particularly, the invention pertains to such technique wherein image quality is improved by means of an anisotropic scan trajectory, and an anisotropic field of view (FOV).
As is well known, spiral scanning is an MR imaging method which has been used to achieve much shorter scan times than can be achieved with conventional MR techniques, for a wide range of applications. The basis for such method is a spiral imaging sequence, where data from k-space is acquired in a spiral rather than the more conventional recti-linear trajectory. In a spiral acquisition, the gradients in the logical X- and Y-axes start from zero and increase in amplitude in a quasi-periodic fashion. This has the effect of a trajectory that starts from the center of k-space and spirals out to some maximum value. Such technique is described, for example, in commonly assigned U.S. Pat. No. 5,604,435, issued Feb. 18, 1997 to Thomas K. Foo and Kevin F. King, the inventor herein. Such technique is further described in other references cited in such patent.
In MR imaging generally, the product of scan time and read-out time is proportional to the product of spatial resolution and field of view. Accordingly, the substantially reduced scan time achieved by the spiral scanning technique requires a longer read-out time, if equivalent spatial resolution is to be maintained. Unfortunately, the spins of off-resonant frequencies tend to cause blurring in a spiral scan image, in proportional relationship to read-out time. In some circumstances, the degree of blurring in a spiral scan image resulting from the increased read-out time is quite intolerable.
For a conventional spiral scan, wherein the time required to reach maximum gradient amplitude is negligible, the read-out time T.sub.r is approximated by the expression T.sub.r .apprxeq..pi.D.sup.2 /4 MBr.sup.2. In such expression, D is the FOV diameter, M is the number of spiral interleaves, B is the band width of the associated MR system receiver, and r is resolution element size. From such expression, the effects of alternative measures to decrease read-out time T.sub.r, in order to reduce off-resonant blurring, will be readily apparent. If T.sub.r is decreased either by decreasing D or increasing r, field of view or spatial resolution, respectively, will be reduced. If T.sub.r is reduced by increasing M, i.e., by using more interleaves, the scan time will be increased. If T.sub.r is decreased by using higher receiver band width, the signal to noise ratio (SNR) of the MR system will be decreased. Each of these results is generally undesirable. Accordingly, the off-resonant blurring effects encountered in spiral scanning have, in the past, tended to place a limit on the maximum spatial resolution and minimum scan time which could be achieved with such technique.